Industrial production of ethylene and propylene oxide (PO) reaches 15 and 3 mill tons/year world-wide, respectively, and is steadily growing. The corresponding market for PO alone can be estimated as about $20 billion per year. Major applications of propylene oxide include production of polyethers, polyols (propylene glycol), polyurethanes and propylene-carbonates. Glycol ethers and polyglycol are used as solvents for paints, coatings, inks, dyes and cleaners. Propylene glycol is used for food flavoring, coloring and fragrance oils, industrial and medical lubricants. Polyether polyols are used for making polyurethane foams.
While ethylene epoxidation is industrially implemented as direct oxidation on silver catalysts and reaches more than 90% yield and very high selectivity, production of PO by direct oxidation is typically far less successful, providing yields in the range of only 1% and very low selectivity. Therefore, propylene oxide needs are currently covered at an industrial scale by two more complex indirect processes, a peroxidation process and a hydrochlorination route. In the past, production was covered primarily by the chlorohydrin process; however, the oxidation process is lately gaining in importance.
While direct epoxidation of ethylene on Ag-based catalysts produces only few side products besides epoxide and full combustion products CO2 and H2O and therefore can easily be used as an industrial production process, direct oxidation of propylene produces a large variety of side products, such as propanal, acrolein, allyl alcohol, acetone, acid, dimers and higher polymerization products. Most side products form from various intermediate metallocyclic complexes of adsorbed oxygen with olefin on the catalyst surface. Because propylene oxide is the species that most easily transforms back into an oxygen adsorbate or other surface complex, experimental synthesis conditions typically promote low gas concentration, short contact time (high pressure) and very dry conditions.
Research has been carried out to identify suitable direct oxidation processes for production of PO. However, many of these processes continue to suffer from low yield and selectivity or cost barriers for large scale synthesis. For example, at least one disclosed direct oxidation method is carried out under hydrogen, which can drive high production costs. With some processes, poisoning of catalyst results in decreased performance over time, which can occur rapidly. Photon-enhanced direct epoxidation with a Cu-based catalyst has also been used to produce PO.